Recently, a water absorbent resin is widely used as a main construction material of sanitary materials (absorbent articles) such as disposable diapers, sanitary napkins, incontinence pads and the like, in order to absorb body fluids (e.g. urine, blood, and the like). Well-known examples of the water absorbent resin are (i) cross-linked partially neutralized polyacrylic acid; (ii) a hydrolyzed starch-acrylonitrile graft polymer; (iii) a neutralized starch-acrylic graft polymer; (iv) a saponified vinyl acetate-acrylic ester copolymer; (v) cross-linked carboxymethylcellulose; (vi) hydrolyzed acrylonitrile copolymer or hydrolyzed acrylamide copolymer, or cross-linked acrylonitrile copolymer or cross-linked acrylamide copolymer; (vii) a cross-linked cationic monomer, (viii) a cross-linked isobutylene-maleic acid copolymer; (ix) a cross-linked polymer of 2-acrylamide-2-methylpropanesulfonic acid and acrylic acid; (x) and the like. There has conventionally been needs for a water absorbent resin having the following water absorbent properties: (i) a high absorbency for a aqueous liquid such as a body fluid, (ii) an excellent absorption rate, (iii) excellent liquid permeability, and (iv) excellent gel strength of a swollen gel, and (v) an excellent absorptive capacity when water is absorbed from a base material containing a aqueous liquid, (vi) and the like.
Each of these water absorbent resins has an even cross-linking structure in the polymer, and is water-insoluble. Generally, in order to attain the foregoing water absorbent properties, surfaces of particles of the water absorbent resin are further cross-linked by using a cross-linking agent or the like, thereby causing the particles to have a cross-linking density gradient. Thus, (i) a water-absorption rate of the water absorbent resin is improved, (ii) generation of fish eye is prevented, (iii) gel strength is improved, (iv) an absorbency of the water absorbent resin under pressure is improved, (v) gel blocking is prevented, and (vi) liquid permeability is improved.
For example, surface cross-linking processes for causing a vicinity of particle surfaces of the water absorbent resin to have a cross-linking density gradient are described in European Patent No. 0349240, European Patent No. 0605150, Japanese Publication for Unexamined Patent Application, Tokukaihei 7-242709, Japanese Publication for Unexamined Patent Application, Tokukaihei 7-224304, U.S. Pat. No. 5,409,771, U.S. Pat. No. 5,597,873, U.S. Pat. No. 5,385,983, and the like (hereinafter, referred to as Prior Art 1).
In addition to the foregoing methods recited, a water absorbent including a water absorbent resin and metal soap in order to improve liquid permeability, is described in Japanese Publication for Unexamined Patent Application, Tokukaisho 61-58658. Moreover, there are needs for such a water absorbent resin which not only has the foregoing water absorbent properties, but also has the following advantages: The water absorbent resin has excellent powder fluidity in a dry state and excellent powder fluidity in a moisture absorption state at the time of production and transportation of the water absorbent resin, at the time of production of an absorbent core by processing the water absorbent resin and a fiber base material or the like, and at the time of moisture absorption, so that the water absorbent resin rarely adheres to a device or the like. As an attempt to produce water absorbent resin having excellent fluidity at the time of moisture absorption, a water absorbent in which an inorganic substance such as amorphous silicon dioxide, kaoline, or the like is added, is proposed. Specifically, for example, art related to a water absorbent including powder of an inorganic substance and powder of a water absorbent resin is disclosed in U.S. Pat. No. 4,734,478, Japanese Publication for Unexamined Patent Application, Tokukaisho 59-80458, and U.S. Pat. No. 5,453,323. In addition to the technique in which an inorganic substance is added, a water absorbent agent in which stearic acid and powder of an inorganic substance are added as additives is described in Japanese Publication for Unexamined Patent Application, Tokukaisho 63-105064 (hereinafter, referred to as Prior Art 2).
Further, a water absorbent agent in which quaternary ammonium salt is added as additives is described in U.S. Pat. No. 5,728,742. Moreover, a particulate water absorbing agent in which denatured polysiloxane, polyethyleneglycol, polypropyleneglycol, or the like are added is disclosed in Japanese Publication for Unexamined Patent Application, Tokukaihei 9-136966 and European Patent No. 0001706. Further, a particulate water absorbing agent containing a polymer dispersant is disclosed in Japanese Publication for Unexamined Patent Application, Tokukaihei 8-143782 (hereinafter, referred to as Prior Art 3).
Moreover, in order to prevent generation of the fish eye and gel blocking, a particulate water absorbing agent obtained by treating a water absorbent resin with a surfactant is disclosed in Japanese Examined Patent Publication, Tokukohei 6-39485 (Published on May 25, 1995), Japanese Patent No. 3283570 (Registered on Mar. 1, 2003), Japanese Publication for Unexamined Patent Application, Tokukai 2003-82250 (Published on Mar. 19, 2003) (hereinafter, referred to as Prior Art 4). Further, an attempt to improve the bulk density is proposed. For example, Japanese Publication for Unexamined Patent Application, Tokukai 2000-302876 (Published on Oct. 31, 2000) recites such arrangement that: indefinite-shape crushed water absorbent resin powder, having large bulk density, whose absorbency against pressure is favorable, is obtained by cross-linking surfaces of cross-linked polymer particles that have been ground (hereinafter, referred to as Prior Art 5).
However, each of the foregoing methods raises such problems that: it is impossible to obtain sufficient fluidity in a dry state and sufficient powder fluidity at the time of moisture absorption; the absorbency against pressure significantly drops; a surface tension of absorbed liquid such as urine drops in case where the water absorbent resin comes in contact with the absorbed liquid. For example, in the Prior Art 1, the fluidity in the dry state and the fluidity at the time of moisture absorption are insufficient, and the water absorbent resin is hard to treat. Further, in the Prior Art 2, by adding an inorganic substance, it is possible to improve the fluidity at the time of moisture absorption, but the fluidity and the absorbency in the dry state significantly drop, so that the water absorbent resin is hard to treat. As a result, an absorbent article using the same cannot sufficiently exhibit water absorbent properties. Particularly, in the technique recited in Tokukaisho 61-58658, generally 1 to 10 wt %, preferably 5 to 80 wt % of metal soap is added to the water absorbent resin. In case where such a large quantity of metal soap is added, its properties such as a hydrophobic property, a water-repellent property, and a surface-activating property have bad influence on the absorbent properties of the water absorbent resin. Further, the Prior Art 3 raises such problems that: the fluidity at the time of moisture absorption is insufficient; a surface tension of absorbed liquid such as urine drops in case where the water absorbent resin is contact with the absorbed liquid. When the water absorbent resin is used in a disposable diaper for example, the water absorbent property of the water absorbent article drops (increase in its re-wet amount (an amount of absorbed liquid which has returned from the absorbent core) or a similar problem occurs). In this way, the water absorbent properties are not sufficient.
Further, in the Prior Art 4, an amount of the surfactant used in the water absorbent resin is large. This is not economical. Further, in case where the particulate water absorbing agent is in contact with absorbed liquid such as urine, a surface tension of the absorbed liquid drops. As a result, such arrangement has bad influence on the water absorbent properties of the particulate water absorbing agent (increase in its re-wet amount or a similar problem occurs) in case where the particulate water absorbing agent is used in a disposable diaper for example.
Further, as to the indefinite-shape crushed water absorbent resin powder in the Prior Art 5, its bulk density is high and its absorbency against pressure is favorable. In order to achieve these properties, the surface of the cross-linked polymer is ground, so that the number of steps required in producing power increases. Further, it is necessary to remove fine powder which has occurred in grinding the surface of the cross-linked polymer. Thus, cost and troubles taken in the production increase.
The present invention was made in view of these problems, and an object of the present invention is easily realize a particulate water absorbing agent having such properties that: (i) the fluidity at the time of moisture absorption and the powder fluidity in the dry state are favorable, so that it is easy to treat the water absorbent resin at the time of transport, which causes the water absorbent resin to less adhere to a production device; (ii) an absorbency against pressure is high; (iii) the water absorbent resin has a superior water absorbent property which prevents a surface tension of absorbed liquid such as urine from dropping in case where the water absorbent resin is in contact with the absorbed liquid; and (iv) the fluidity and the bulk density of the water absorbent resin are stably increased without losing the water absorbent property.